This invention relates to a power transmission apparatus, a power reception apparatus and a power transmission system, and particularly to a power transmission apparatus, a power reception apparatus and a power transmission system which can transmit power contactlessly in small equipment.
In recent years, research and development of a system for transmitting electric power contactlessly has been conducted. One of such systems is disclosed, for example, in Japanese Patent Laid-Open No. 2008-295191.
As a power transmission method for contact less power transmission system such as an electromagnetic induction type power transmission method is available. Further, in recent years, a power transmission method a magnetic field resonance type power transmission method has become available. The magnetic field resonance type power transmission method allows for the transmission of power over a long distance in comparison to an electromagnetic induction type power transmission method.
FIG. 1 shows an example of a configuration of an existing power transmission system to which the magnetic field resonance type power transmission method is applied.
The existing power transmission system 11 shown in FIG. 1 consists of a power transmission apparatus 21 and a power reception apparatus 22.
The power transmission apparatus 21 includes an oscillation circuit 31, a power transmission coil 32 and a resonance circuit 33 which are accommodated in a single housing 21A.
The power reception apparatus 22 includes a resonance circuit 51, a power reception coil 52, a bridge rectification circuit 53 and a smoothing capacitor 54 which are accommodated in a single housing 22A.
The existing power transmission system 11 having such a configuration as described above operates in the following manner.
In particular, alternating current outputted from the oscillation circuit 31 flows to the power transmission coil 32, and as result, an oscillating electromagnetic field is generated around the power transmission coil 32. Alternating current is induced by the oscillating electromagnetic field of the power transmission coil 32 and flows to the resonance circuit 33 on the power transmission side, and as a result, an oscillating electromagnetic field having a predetermined resonance frequency is generated around the resonance circuit 33 on the power transmission side.
The resonance circuit 51 on the power reception side of the power reception apparatus 22, alternating current flows by resonance of the oscillating electromagnetic field of the resonance circuit 33 on the power transmission apparatus 21 side. In particular, wireless non-radiation type energy transfer is carried out using an electromagnetic field mode of oscillation resonance so that alternating current flows to the resonance circuit 51 on the power reception side. As a result, an oscillating electromagnetic field having a predetermined resonance frequency is generated around the resonance circuit 51 on the power reception side. Alternating current is induced by the oscillating electromagnetic field of the resonance circuit 51 on the power reception side and flows to the power reception coil 52. This alternating current is full-wave rectified by the bridge rectification circuit 53. The full-wave rectified current in the form of pulsating current is smoothed by the smoothing capacitor 54 and then supplied to a circuit on the following stage not shown.
In this manner, in the existing power transmission system 11, power is supplied contactlessly from the power transmission apparatus 21 to the power reception apparatus 22.
Incidentally, in such a magnetic field resonance type power transmission method applied to the existing power transmission system 11 as described above, if the Q value of the resonance circuit is not raised, then the transmission efficiency cannot be enhanced. In particular, in the example of FIG. 1, in order to enhance the transmission efficiency, it is necessary to set the Q value of the resonance circuit 33 on the power transmission side and the resonance circuit 51 on the power reception side to a high value.
It is to be noted that, since, with such a frequency as is utilized in the magnetic field resonance type power transmission method, the Q value of the resonance circuit depends upon a characteristic of a coil, the Q value is calculated in accordance with the following expression (1):
                    Q        =                  ω          ⁢                      L            R                                              (        1        )            where ω is the angular frequency, L the inductance value of the coil of the resonance circuit and R the resistance value of the resonance circuit.
FIG. 2 illustrates an example of variation of the transmission efficiency of power by the magnetic field resonance type power transmission method.
In FIG. 2, the axis of ordinate indicates an attenuation amount [dB] with respect to a maximum transmission efficiency. The attenuation amount represents a transmission efficiency. The axis of abscissa indicates an oscillation frequency [MHz] of the oscillation circuit (in the example of FIG. 1, the oscillation circuit 31) on the power transmission side.
It is to be noted that, in one embodiment of FIG. 2, the resonance frequency is 13.56 MHz of the ISM (Industrial, Scientific, Medical) band. In another embodiment of FIG. 2, the resonance frequency is set to 120 kHz. Further, in the example of FIG. 2, a very high value of approximately 400 is adopted as both Q values.
As shown in FIG. 2, where the oscillation frequency is 13.56 MHz which is equal to that of the resonance frequency, the transmission efficiency is highest and the attenuation amount is zero.
However, it is difficult to apply the existing power transmission system 11 as a power supply to small equipment such as a portable telephone set, an electronic notebook, a headphone, a music player and the like.
In particular, such small equipment frequently is used at a place spaced from a power supply by several meters or more. As a result, efficient power transmission at a transmission distance of several meters or more is required for the existing power transmission system 11. In order to satisfy the request just described where the existing power transmission system 11 wherein 13.56 MHz described above is used as the resonance frequency is applied, it is necessary to increase the diameter of the coils of the resonance circuit 33 on the power transmission side and the resonance circuit 51 on the power reception side to approximately 0.44 m. It is very difficult to accommodate such a large coil having a diameter of approximately 0.44 m as just described in the inside of a small equipment.
Therefore, it is desirable to implement contact less electric power transmission in a small equipment.